"I know you're a trial lawyer, and in my trial experience, I've never seen an offer of proof like this," Justice Paul Anderson told Coleman lead attorney Joe Friedberg. "It doesn't identify witnesses. It doesn't identify what they're going to say in terms of what exhibit that they're challenging. … Why is this offer of proof not inadequate, in that we don't have admissible evidence that can be considered by this court in determining whether you've met your burden [of proof]?"

Another justice, Christopher Dietzen, told Friedberg: "It seems like you're offering little more than an opening statement of, in this case, Coleman's theory of the case, but no concrete evidence to back it up."

During Monday's oral arguments, both sides had 25 minutes to present their case and answer justices' questions. Coleman's side was then given an additional 10 minutes to rebut Franken's case.

Coleman's lawyers told the justices that different counties used different standards to decide which ballots to count. Moreover, they said the three-judge election panel imposed a tougher standard for which ballots to count, effectively changing the rules of the election in the middle of the game.

Friedberg asked justices to "extrapolate" that more ballots would have been included, had the counties all applied the same standard. Coleman wants several thousand more ballots tallied.

Franken's lead attorney, Marc Elias, also faced close scrutiny, but did not appear to be on the hot seat the way Friedberg clearly was.

Elias conceded that no election is perfect, but said Coleman's team had fallen short of proving that voters' rights of due process and equal protection under the law had been violated – the grounds upon which the Coleman case rests.

Elias told justices that "every ballot has a story," meaning that local and state election officials knew the circumstances of the ballots when they decided whether to count them. Ballots that appeared invalid based on state law – absentee ballots without the required signature, for example, or where the voter's registration status could not be verified – could simply be common sense exceptions, Elias suggested.

One of the key questions for justices is whether state officials and judges applied a "strict compliance" standard, or a "substantial compliance" standard, in determining which ballots to count.

Strict compliance means a ballot is invalidated whenever it fails to meet the stipulated requirements of the law. Substantial compliance means that if a voter's intent is clear, the vote can be counted even when it could perhaps be disqualified on technical grounds.

Coleman's legal team is arguing that different standards were applied at different times and places, thereby violating the constitutional rights of voters whose ballots were not tabulated.

Legal scholars caution that tough questions don't always indicate which way the justices may be leaning. Some judges, they say, direct tougher questions to the side whose arguments they find more convincing, in order to test the soundness of those points, and to avoid any appearance of favoritism.

After the oral arguments were presented, Coleman, who attended the oral arguments, told the media: "I don't know what's in those [uncounted] ballots. But every one of those ballots would have been counted if they lived in a different area. And whether your vote counts or not shouldn't depend on where you live."

Friedberg emphasized that Coleman merely seeks to ensure that every valid vote is counted.

"It's really strange that the counties with the resources, like Minneapolis' Ramsey County and St. Louis County -- the Democratic strongholds -- are the ones that relaxed the standards and let the higher percentage of ballots in," Friedberg said. "While the good old Republican counties followed the rules. We pay for that now."

Friedberg did not appear shaken or concerned by the justices' questions.

"Those questions were right on. There wasn't a single unfair question to either side," Friedberg said.

Coleman's attorneys are asking the state's highest court to send the case back to the three-judge panel that heard the election contest originally, ordering them to count absentee ballots that court had refused to consider.

The race has drawn national attention because a Franken win would give Democrats their 60th vote for in the Senate, theoretically giving them a veto-proof majority.

There has been speculation that Minnesota Gov. Tim Pawlenty would come under very strong political pressure – and possibly legal pressure as well -- to sign a certificate of election, even if Coleman declares his intention to appeal to the Minnesota Supreme Court's ruling. Pawlenty has stated that he will sign the certificate for whoever emerges the winner, when he is directed to do so by law.

Three of the five justices hearing the appeal were appointed by Republicans, and a fourth was appointed by former Gov. Jesse Ventura, an independent. A spokesman for Minnesota Republican party has told Newsmax that state's GOP leaders have generally confidence that the justices will give Coleman a fair hearing.

It could be days or weeks before the Minnesota Supreme Court rules on the appeal. Coleman and his attorneys declined to state Monday whether they would file a federal appeal, perhaps including one with the U.S. Supreme Court, appeal if Minnesota's highest court rules in Franken's favor.